1. Field of the Invention
The present invention relates to a dielectric ceramic and a laminated ceramic capacitor, and more particularly, relates to a dielectric ceramic which is suitable for reducing the size of and increasing in the capacitance of a laminated ceramic capacitor, and to a laminated ceramic capacitor composed with the use of the dielectric ceramic.
2. Description of the Related Art
In dielectric ceramics used for constituting the dielectric ceramic layers provided in laminated ceramic capacitors, BaTiO3 based compounds have been used as main constituents of the dielectric ceramics in order to have a high dielectric constant. In particular, (Ba, Ca)BaTiO3 obtained by the substitution of some Ba of BaTiO3 with Ca can provide high reliability (high temperature load life characteristics) and favorable capacitance temperature characteristics.
Extremely severe demands have been made on laminated ceramic capacitors in recent years for a reduction in size and an increase in capacitance. For example, the demand for reduction down to 1 μm or less on the thickness of the dielectric ceramic layer has been made. As a result, the strength of the electric field applied to the dielectric ceramic layer has been increasing, thereby leading to increasingly severe design requirements for ensuring reliability.
As a means for solving the problem described above, for example, Japanese Patent Application Laid-Open No. 2005-194138 (Patent Document 1) proposes the addition of predetermined amounts of various elements such as V in a dielectric ceramic.
The dielectric ceramic proposed in Patent Document 1 is, more specifically, represented by the composition formula: 100(Ba1-xCx)mTiO3+aMnO+bV2O5+cSiO2+dRe2O3 (where the Re is at least one metal element selected from among Y, La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb), and the x, m, a, b, c, and d, respectively, satisfy conditions of 0.030≦x≦0.20, 0.990≦m≦1.030, 0.010≦a≦5.0, 0.050≦b≦2.5, 0.20≦c≦8.0, and 0.050≦d≦2.5.
However, it has been found that the dielectric ceramic just described has the following problems. Since this dielectric ceramic shows large variations in grain diameter, it has been found that in the case of using the dielectric ceramic in a laminated ceramic capacitor, the dielectric ceramic has the problems of a shortened high temperature load life (i.e., reliability) and larger variations in life when an high electric field with its strength of, for example, 15 kV/mm or more is applied while reducing the thickness of the dielectric ceramic layers to on the order of 1 μm.